Composites of ceramic fiber and flurocarbon polymer, particularly polytetrafluoroethylene (PTFE) polymer, are known and are highly advantageous for use as friction material due to high temperature and corrosion resistance, particularly to greases, oils and fuel, in conjunction with having the ability to maintain a substantially constant surface friction factor under widely varying conditions.
An example of a friction facing material having proportedly superior dynamic frictional torque transmitting characteristics that comprises a composite of flurocarbon polymer and carbon particulates, and which may optionally include ceramic fibers is disclosed in U.S. Pat. No. 4,593,802, the disclosure of which is incorporated herein by reference.
Another example of a material believed to be a composite of ceramic fibers and polytetrafluoroethylene (PTFE) polymer is sold under the Trademark "Gylon" by the Garlock Company.
As used herein, the term "ceramic fibers" include fibers of silicon nitride, silicon carbide, and alumina-silicate fibers well known to those skilled in the art of ceramic fibers as well as including mixtures thereof.
As used herein the term "flurocarbon polymer" means those flurocarbon polymers whose crystallinity can be increased by controlling cooling rates from sintering or annealing temperatures and includes polytetrafluoroethylene (PTFE) polymer, fluorinated ethylenepropylene copolymer (FEP), and perfluoroalkoxy copolymer (PFA) of which PTFE, FEP and PFA are sold under the Trademark "Teflon" by the Dupont Company and (PTFE) under the Trademark "Halon" by Allied Corporation.
Also known is the effect that cooling rate has upon crystallinity of PTFE polymer after having been sintered at a temperature of from 340.degree. C. to 440.degree. C. for a prescribed period of time and after having been annealed at a temperature of from 323.degree. C. to 323.degree. C. for a prescribed period of time as reported by P. E. Thomas et al of the Dupont Company in an article titled "Properties of Teflon Resins" beginning page 89 in the June, 1956 SPE Journal. Generally, the article describes a substantial increase in crystallinity ranging from about 44% when the PTFE is water or air quenched after sintering to as high as 80% where it is cooled at a rate of 2.degree. C./minute after sintering.
The article further discloses that although flexural modulus can be increased as much as five fold and compressive stress as much as 50% by increasing its crystallinity, compressibility can be reduced by as much as 50% and percent recovery after compression as much as 10%.
Thus, in view of the above, it would appear that increasing the crystallinity of PTFE would have a detrimental effect upon the ability of PTFE in conjunction with ceramic fibers to provide an effective frictional material involving repeated engagements where some degree of compression is involved due to its loss of resiliency and inability to recover completely or substantially completely from the compression.
It has been surprisingly discovered that, contrary to the teaching relative loss in resiliency and inability to recover completely or substantially completely from compression, increasing the crystallinity of a flurocarbon polymer such as PTFE by controlling the rate of cooling after a sintering or annealing operation actually enhances its effectiveness in conjunction with ceramic fibers as a frictional material particularly in vehicular applications.